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<title>Boost Endian Library</title>
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<td width="277">
<a href="../../../index.html">
<img src="../../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86" border="0"></a></td>
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<font size="7">Endian Library</font></td>
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<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%">
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<td><a href="../../../index.htm">Boost Home</a>&nbsp; Conversion Reference
Types Reference Tutorial</td>
</tr>
</table>
<h2><a name="Introduction">Introduction</a></h2>
<p>The Boost Endian Library provides facilities to deal with endianness. See
<a href="#Introduction-to-endianness">Introduction to endianness</a> below for
the basics of endianness.</p>
<p>The library provides two approaches to dealing with integer endianness:</p>
<blockquote>
<p><b>Endian conversions for native integers -</b> The application uses the
built-in integer types, and calls the provided conversion functions to swap
bytes as needed. Both mutating and non-mutating conversions are supplied, and
each comes in unconditional and conditional variants. This approach is simple
and efficient, but is less flexible in terms of size and alignment, and can be
hard to manage in code with many logical paths involving endianness transitions.</p>
<p><b>Endian integer types</b></p>
</blockquote>
<h2><a name="Introduction-to-endianness">Introduction to endianness</a></h2>
<p>Consider a C++ program that defines variables x, y, and z as 16, 32, and
64-bit integers, respectively. There are several ways a processor might layout
the individual bytes for these variables in memory:</p>
<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
<tr>
<td colspan="6" align="center"><b>int16_t x = 0x0A0B;</b></td>
</tr>
<tr>
<td colspan="3" align="center"><b>Big Endian</b></td>
<td colspan="3" align="center"><b>Little Endian</b></td>
</tr>
<tr>
<td><b>Value</b></td>
<td align="center"><b>0A</b></td>
<td align="center"><b>0B</b></td>
<td><b>Value</b></td>
<td align="center"><b>0B</b></td>
<td align="center"><b>0A</b></td>
</tr>
<tr>
<td><b>Address</b></td>
<td align="center"><b>...0</b></td>
<td align="center"><b>...1</b></td>
<td><b>Address</b></td>
<td align="center"><b>...0</b></td>
<td align="center"><b>...1</b></td>
</tr>
</table>
<br>
<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
<tr>
<td colspan="10" align="center"><b>int32_t y = 0x0A0B0C0D;</b></td>
</tr>
<tr>
<td colspan="5" align="center"><b>Big Endian</b></td>
<td colspan="5" align="center"><b>Little Endian</b></td>
</tr>
<tr>
<td><b>Value</b></td>
<td align="center"><b>0A</b></td>
<td align="center"><b>0B</b></td>
<td align="center"><b>0C</b></td>
<td align="center"><b>0D</b></td>
<td><b>Value</b></td>
<td align="center"><b>0D</b></td>
<td align="center"><b>0C</b></td>
<td align="center"><b>0B</b></td>
<td align="center"><b>0A</b></td>
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<td align="center"><b>...0</b></td>
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<td align="center"><b>...2</b></td>
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<td align="center"><b>...0</b></td>
<td align="center"><b>...1</b></td>
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<br>
<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<td colspan="18" align="center"><b>int64_t z = 0x0A0B0C0D0E0F0102;</b></td>
</tr>
<tr>
<td colspan="9" align="center"><b>Big Endian</b></td>
<td colspan="9" align="center"><b>Little Endian</b></td>
</tr>
<tr>
<td><b>Value</b></td>
<td align="center"><b>0A</b></td>
<td align="center"><b>0B</b></td>
<td align="center"><b>0C</b></td>
<td align="center"><b>0D</b></td>
<td align="center"><b>0E</b></td>
<td align="center"><b>0F</b></td>
<td align="center"><b>01</b></td>
<td align="center"><b>02</b></td>
<td><b>Value</b></td>
<td align="center"><b>02</b></td>
<td align="center"><b>01</b></td>
<td align="center"><b>0F</b></td>
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<p>The most-significant byte first ordering is traditionally called &quot;big endian&quot;
ordering and the least-significant byte first is traditionally called
&quot;little-endian&quot; ordering. Although some other orderings are possible, most
modern uses are as shown above. The names are derived from
<a href="http://en.wikipedia.org/wiki/Jonathan_Swift" title="Jonathan Swift">
Jonathan Swift</a>'s satirical novel <i>
<a href="http://en.wikipedia.org/wiki/Gulliver's_Travels" title="Gulliver's Travels">
Gulliver<EFBFBD>s Travels</a></i>, where rival kingdom's opened their soft-boiled eggs
at different ends.</p>
<p>Intel processors are traditionally little endian while many others are big
endian. Some processors can switch endianness, so which is in use depends on the
operating system. The Wikipedia's
<a href="http://en.wikipedia.org/wiki/Endianness">Endianness</a> entry lists
details for many processors and operating systems.</p>
<p>External memory, such as disks, generally uses the same endianness as the
operating system. Networks traditionally use big endian ordering, so this is
sometimes referred as network endianness.</p>
<hr>
<p>Last revised:
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->27 May, 2011<!--webbot bot="Timestamp" endspan i-checksum="13974" --></p>
<p><EFBFBD> Copyright Beman Dawes, 2011</p>
<p>Distributed under the Boost Software License, Version 1.0. See
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a></p>
<p>&nbsp;</p>
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@@ -653,11 +653,10 @@ Tomas Puverle, Vincente Botet, and
Yuval Ronen.</p>
<hr>
<p>Last revised:
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->25 March, 2009<!--webbot bot="Timestamp" endspan i-checksum="29032" --></p>
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->27 May, 2011<!--webbot bot="Timestamp" endspan i-checksum="13974" --></p>
<p>© Copyright Beman Dawes, 2006-2009</p>
<p>Distributed under the Boost Software License, Version 1.0. (See accompanying
file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a>)</p>
<p>Distributed under the Boost Software License, Version 1.0. See
<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a></p>
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<p><font size="2"><EFBFBD> Copyright Beman Dawes, 2001</font></p>
<p><font size="2">Distributed under the Boost Software License, Version 1.0.
(See <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)
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